This invention is a device for preventing unauthorized opening of the roll-up door of a cargo container of the type used on truck trailers or roll-up doors of garages, warehouses, or mini-storage warehouses. The roll-up door of such installations is customarily secured with rollers that slide or roll within glides that are mounted within the door frame on laterally opposite sides of the rear of the trailer or the entrance way to the garage or mini-storage. The doors have hinged horizontal panels that fold as the door is opened by being slid, customarily in an upward direction, along glides that typically turn inward of the trailer or garage at an angle approximating ninety degrees. Although various locking devices have been proposed for securing roll-up doors, thieves using bolt cutters, power hack saws and sledge hammers are all too frequently destroying the locking devices that are mounted externally on these doors.